The present invention relates to a control arrangement for regulating the speed of a pressure cylinder, particularly of a preacceleration cylinder in a drawing apparatus of the drawing stage of a press. The arrangement has a control valve in the delivery pipe leading to the pressure cylinder which can be adjusted in its cross flow section.
Pistons and piston rods of pressure cylinders must be moved at various and different speeds. The controlling of the speeds takes place by valves which can be changed in their (flow) cross section, for example, by means of proportional-action valves.
The flow characteristics of such valves show that a regulating of the flow (of the flow rate) can take place only within limits, relative to the opening cross section. Ranges of 0% to approximately 30% of the opening cross section and of approximately 60% to 100% of the opening cross section have almost no effect on the regulating or control. After the cross section is increased to, for example, 60%, no significant volume flow increases take place. Lower flow ranges may be regulated by means of one valve; higher flow ranges that can be connected require coupling in parallel. Valves with larger flow cross sections,, in turn, are unsuitable for lower flow ranges. Parallel connections of control valves require high control expenditures also because of the required isochronism of the regulating of the flow cross sections.
Parallel connections have been used, for example, in cases where pressure cylinders must be controlled in rapid motion (high flow volumes) and in one operating cycle (high pressure). See, for example, Maschinenmarkt, Wurzburg 88 (1982) 57, Page 1177 to 1180.
The German Patent Document DE 36 40 788 A1 discloses a drawing device in a press comprising a preacceleration cylinder. The present invention will be explained in the following by using the pressure cylinder that is shown in this patent document as an example, but the invention is not limited to using this type of cylinder. In the case of drawing devices of the disclosed type, a holding force must be applied for the drawing of a metal sheet. The drawing slide moves the metal sheet and a blank holder against the holding force applied by the drawing device. In addition, forces of the inertia of masses, which occur at the start of the deformation, must be overcome. In this case, forces occur abruptly which are reduced by the preacceleration of the blank holder with the metal sheet resting on it in the drawing direction. The preacceleration is generated via a cylinder which is acted upon by pressure. As a function of the number of strokes of the press, the quantity of hydraulic fluid must be controlled that flows to the pressure cylinder. The pilot control pressure cylinder, which feeds a hydraulic fluid to the preacceleration cylinder in a targeted manner, can be connected with a pressure source by a proportional-action valve. However, in this case, the speed range of the pilot control pressure cylinder is narrowed down by saturation ranges of the proportional-action valve (=30%; 60% to 100%).
It is therefore an object of the invention to provide a control arrangement by which, on the one hand, a larger valve is avoided for the control of a pressure cylinder and by means of which, on the other hand, parallel connections of valves are avoided for a flow volume that is controlled in a targeted manner.
This and other objects are achieved by the present invention which provides a control arrangement for regulating the speed of a pressure cylinder in a drawing apparatus of a drawing stage of a press. This control arrangement has a control valve in a delivery pipe leading to the pressure cylinder. The control valve is adjustable in its flow cross section. The control valve has at least first and second flow paths connected in parallel. At least one controllable valve is connected in series to the first flow path, for blocking or releasing of the first flow path, the thus controllable first flow path and the controllable valve being connected in parallel to the second flow path.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.